1. Field of the Invention
This invention relates to a multiple feed detecting system for detecting multiple feed of sheets, that is, that a plurality of sheets are conveyed along a conveyance passage one on another, in printers, copiers or the like.
2. Description of the Related Art
For example, a paper feed mechanism in printers separates and conveys sheets one by one from a sheet stack. However a plurality of sheets sometimes can be fed to the printing drum superposed or overlapped one on another. This phenomenon will be referred to as "multiple feed", hereinbelow. In order to accurately detect such multiple feed, sampling is generally effected over a wide area of the sheet to be fed.
FIG. 19 shows a light transmission type multiple feed detector provided to the paper feed mechanism. The multiple feed detector 1 comprises a light emission sensor 1a and a light reception sensor 1b opposed to each other intervening therebetween a sheet 11. The sheet 11 is disposed in the middle between the sensors 1a and 1b preferably in perpendicular to the line joining the sensors 1a and 1b so that change in transmittance due to reflected light or the like can be suppressed.
When detecting multiple feed of the sheets 11 by the above arrangement, there is an area of the sheet 11 which is not suitable for sampling. FIG. 20 shows a state where the sheet 11 is slackened by resist rollers 9 at the beginning of feed shown in FIG. 19. In the state shown in FIG. 20, the sheet 11 cannot be positioned in the middle between the sensors 1a and 1b and sometimes cannot be in perpendicular to the line joining the sensors 1a and 1b. FIG. 21 shows a state where the sheet 11 is released from a pickup roller 8 at the end of feed. Also in this state, the above problem occurs.
Thus, the leading end portion and the trailing end portion of the sheet 11 are instable in position relative to the detector 1 and accordingly are not suitable for sampling for detecting multiple feed. When such a portion is sampled, detecting accuracy deteriorates.
When sampling only an intermediate portion of the sheet 11, the sheet size gives rise to a problem. FIG. 22 shows the sampling timings of the light reception sensor 1b when an A-3 sheet and an A-5 sheet are fed by the paper feed mechanism shown in FIG. 19.
Sampling the A-5 sheet over the sampling area for the A-3 sheet results in sampling a vacant area. To the contrast, sampling the A-3 sheet over the sampling area for the A-5 sheet results in sampling over an area narrower than available.